Marine seismic surveying



Oct. 14, 1952 w. w. DOOLITTLE 2,614,165

MARINE SEISMIC SURVEYING Original Filed Feb. 23, 1949 2 SHEETS-SHEET 1FIE-L5 q. vm

INVENTOR. WADDlE W. DOOUTTLE BYMW ATTO R N EY 1952 w. w. DOOLITTLEMARINE SEISMIC SURVEYING 2 SHEETSSHEET 2 Original Filed Feb. 23, 1949INVENTOR. WADDIE W. DOOLITTLE ATTORNEY Patented Oct. 14, 1952 MARINESEISIWIC SURVEYING Waddie W. Doolittle, Picayune, Miss., assignor toStanolind Oil and Gas Company, Tulsa, Okla., a corporation of DelawareOriginal application February 23, 1949, Serial No. 77,776. Divided andthis application October 17, 1949, SerialNo. 122,674

4 Claims.

This invention relates to geophysical surveying and is directedparticularly to prospecting by seismic methods over water-covered areassuch as in the Gulf of Mexico. This application is a division ofapplication Serial No. 77,776, filed February 23, 1949.

Geophysical surveying using artificially created seismic waves has beenextensively and successfully used on land for anumber of years, but itis only comparatively recently that the method has been applied tooff-shore exploration for oil and gas in the Gulf of Mexico. In theearliest applications of the seismic method to marine areas the shotsand detectors were individually placed on or under the marine floor inmuch the same manner as in prospecting on land. The results obtainedwere generally similar to those obtained on land prospects.

With the improvement and adaptation of specific techniques andinstruments for this marine work, the speed of prospecting by thismethod has increased so markedly over what was previously possibleeither on land or water that, as a result, more than the normal ratio ofgeophysical effort in marine prospecting has been concentrated on theseismic method, as compared with magnetic and gravimetric methods, forexample.

Both now and in the past one of the difli'cult problems connected withthis method has been the proper handling of the seismometers used fordetecting the seismic waves. Placing the seismometers at known locationsin a spread on the marine floor as in land prospecting, proved even morelaborious and time-consuming than on land. Towing a spreadofseismometers connected together by a conductor and tension cable alongthe marine floor or supported by floats behind the recording vessel fromone location to another, and shooting either with the seismometers onthe marine floor or supported from the floats have resulted in a'markedincrease in the speed of carrying out the geophysical surveys. However,dragging of the seismometer spread along the sea bottom presents obviousdisadvantages in the hazards both to the equipment and to the marinelife and installations located on the marine floor. Employingseismometers at or'near the surface and supported by floats results inthe picking up of a great deal of noise, even under favorable conditionsof .low wind velocity and relatively smooth water surface. Even in calmseas the noise picked up by float-supported; near-surface detectors'issuch as tomask'most'of the desired weak reflections;

while on windydays "and-when the water surface is rough, the noise maybe; so strongas to override all reflections and make prospectingimpossible. As a matter offact, there are'some seasons'of the year whenthe noiseconditions from the water surface have'been sotroublesome thatprospecting operations in the Gulf of Mexico were possible only a smallfraction ofthe time. The resultant .delays' while the crewsand equipmentare held in readiness for favorable working conditions add verygreatlyto the expense of the operation.

It is'accordingly a primary object of myinvention to provide a methodand apparatusfor marine seismograph prospecting which gives a greatlyimproved signal-to-noise ratio permitting detection of deeper and weaker'desired reflection signals. Another object is to. provide a marineprospecting method and apparatus in which'the seismometer spread depthis controlled or varied so as to bring the seismometers to the'most ef-'fective depth for receiving signals. A further object is to provide amarine seismograph prospecting method and apparatus which can operateand obtain good geophysical data under adverse conditions of weatherandwater such that usable data could not hitherto beobtained. Still anotherobject is to provide a towable marine seismometer spread in which thedepth of submergence of the seismometers is float-controlled from thewater surface, butin a manner which minimizes the transmission ofnoisesignals to the seismometers. A still further object is 'to providea marine seismometer spread which creates a relatively small drag on thetowing vessel thereby reducing the travel time between shot points' andincreasing the speed of the prose pecting operation. Still anotherobject is to provide a marine seismometer spread which 'may be towedclose tothe water surface between shot points, but at shot pointsreadily. lowers the seismometers. to the desired depth. Still anotherand further object of my invention is to provide a marine seismometerspread having good discrimination or filtering against the transmissionof longitudinal vibrations along the towingand conductor cable. Otherand further objects; uses; and advantagesof the invention willbecomeapparent as the description proceeds.

From observations made under a variety of conditions, it has now beenfound that the range of depths in water where seismometers maybe placedfor the most eificient operation is relatively narrow. Due to thegreatly different seis mic-wave transmission properties of water and thewater surface.

3 air, seismic waves traveling upward from the earth below, through thewater to its surface, are almost totally reflected there. As a result,there is a strong probability of interference between succeeding wavesin a train of seismic waves at a depth in the water which is onequarterof the average seismic wave length in that medium. To avoid thispossible interference,

, towing slows down or comes to a stop at a which changes the characteror appearance of the detected waves, it is therefore desirable to locatethe seismometers as close as possible to It is at this surface that 'thedisplacements are a maximum, and the possibility of interference is aminimum.

As was briefly indicated above, it has been found that the level ofnoise within the seismicwave band is a maximum at the water surface, andfurther that this noise level drops off very sharply with depth.Accordingly, there is for seismometer operation a narrow range ofdepths, theupper limit of which may be designated as below a "zone ofsurface noise and is determined by, the depth where the noise levelreaches a satisfactorily low value, and the lower limit of which rangeis the depth at which interference effects become pronouncedrfor wavesof interest in'the seismic band. This range extends from about feet toabout feet, with the preferred depth. of operation, at which quiteconsistently good records are obtained, being about 10 feet.

. Althoughthis range accurately represents the presently preferredpractice, operations are still possible above it at the expenseofintroducing intothe recorded signals some noise, which may not.always beobjectionable. A very substantial reduction in the noise level takesplace in the first foot or two of the 5-foot surface noise zone,softhat'm'any of the advantages of the present invention, are retainedwhen operating at such a shallow depth. that it can only be said thatthe seismometers'are substantially-below the water surface. Particularlyis this true in the .absence of surface floats immediately above theseismometers, which floats are themselves sources of .noise .as thewater moves relative to them, becauseof waves or currents,'for example.1

It is conceivable also that some useful results, such as-theemphasis ofcertain reflections, first arrivals, orthe like, might be obtained bydeliberately locating the seismometers at a depth where interference ofa selected wave-length withinfthe seismic band would occur. Thisinvention, offers ,aconvenient way of operating at such depths simply.by increasing either the time or.}the rate of seismometer submergence.

vAccordingly, the foregoing enumerated and other objects areaccomplished in my invention by am'arine seismometer spread which istowed near the water surface between shot points and, at, a desiredlocation for recording, submerges uniformly and slowly to the desireddepth. Upon reaching. this depth theshot is fired to initiate thedesiredsignals, and the record is made. According tothe preferred embodiment ofmy invention, seismometerdepth control is achieved by adjusting ,thetowing cable or cables, seismometers, and. various supporting ,floatsvery closely to aneutralbuoyancy, with the spread as a whole,however,having a smallpositive overall buoyancy. This insures that thespread will be, near the water surface rather than drag againstthe-bottomas it is pulled along behind the towing-vessels However, theseismometers themselves or portions of the cable near themare,slightlynegatively buoyant, so that as the shot point either byslowing the towing vessel or paying out the cable, the seismometersslowly submerge and finally reach the desired operating depth. Thepreferred embodiment of this spread, therefore, includes-a plurality ofspaced seismometers connected by a tension .and electrical conductorcable, all having an approxi- -mately neutral buoyancy. Portions of thecable,

preferably midway, between the seismometers are positively buoyant,while portions between these buoyant sections and either at or near theseismometers are negatively buoyant so as to cause slow submergence asthe forward motion of the spread slows or stops.

This will be better understood by reference to the accompanyingdrawings, forming a part of Q this application, in which like numeralsare applied to the same or corresponding parts in the different figures.In these drawings,

Figure l is a cross-sectionof av body of water through which a spreadembodying the invention is being towed by a vessel and is shown in aposition for. recording; I

Figures2 and 3 are respectively elevation and cross-sectional views of aseismometer-supporting float;

Figure 4 is a cross-section of a housing for a gimbal-suspendedseismometer; 1

Figure 5 is an elevation view of a cable-supporting float;

Figure 6 is a plan view, and Figure '7 is a crosssectiona1view,,of abody of water showing a complete spread and shooting apparatus operatingin accordance with my invention; and r Figure 8 is a cross-section ofabody of water showing in part an alternative embodiment of the inventionin recording position therein.

Referring now to these drawings in detail and to Figure l in particular,a vessel '20 is shown proceeding through a body of water 2| towing aspread 22. constructed in accordance with the invention. Spread 22 ismade upof a cable 23,.

having both strands with a considerable tensile strength for connectingtogether and towing the various components, and insulated electricalconductors for the detector signal leads. Cable 23 is supported by aplurality of floats 24 spaced at intervals along its length. Also spacedalong the cable 23 at any desired intervals, are a number of detectorcontaining supports or, floats 25. The forward'end of cable 23, isattached to the vessel 20, while its trailing end is coupled to a floatordrag 26 which, by opposing the forward motion of the spread, maintainsthe cable in tension during towing. It is. possible, however, toomitdrag 25, as the drag of the spread itself in passing through thewater is often sufficient. drag 26 indicates the position of the end ofthe spread to an observer on vessel 20 so that the spread direction maybeascertained at all times, particularly in the presenceofcross-currents.

In accordance with my invention, the spacing and the buoyancy ofthecable 23, the cable floats 24, and the seismometer floats 25 are soadjusted that the spread has very nearly a neu- However, the par-.

magnitude of-.sui table relative buoyancyfo'rces,

a spread constructed in accordance with my in- A flag 21 .mounted onfloat or i ventionv was. first adjustecl'to have as nearly'as possiblexa.neutral'buoyancy' over-all. The seismometer floats 25' were thenweighted by the addition. of 8 ounces" ofxwei-ght each, whilethe floats.24a=were rendered buoyant to the' extent of '12 ounces each. This gaveasatisfactory positive over-all buoyancy and a seismometer submergencetime of between 1 and 3 minutes after the towing was discontinued.

By synchronizing the placing of the explosive charge with themanipulation of the spread, there was-no difficulty in detonating 'thecharge when, in this time interval, the seismometers reached the properdepth;

It should be notedthat the unsupported portions of cable 23 betweenthefloats 24havea slight amount of 'sag due to the cable-itself beingnegatively buoyant. This is adefinite advantage-in that'theseunsupported cable portions act toreduce the transmission of longitudinalvibrations along the cable, particularly 5 when the cable tension isverysmall, as itis intherecord ing position. In addition, the individualfloats 24 themselves, having a small 'but definite drag which opposesmotion through the water, co-- operate with the unsupported lengths ofcable 23in absorbing longitudinal vibrations: These two effects, plusthe fact that the buoyant portions of cable 23 andfloats 24a which areat or near thewatersurface are located at a considerablelateral-distance from the seismometer floats 25, result in substantiallyzero transmission of surface water noises to the seismometers.

One of these seismometer floats .25 is shown in more detail in Figures 2and 3. As shown in Figure 2, thefloat'is preferably elongated andprovided with pointed ends so-as to be towed easily through the water.It is everywhere cir cular in cross-section to avoid "any tendencies'tofloater dive duringtowing-andso it" streams smoothly directly back fromthe point of applicationof towing force. Thebuoyancy'of float 25 isadjusted by adding or removing small strapsoi closure 35, provides forawaterproof splice andinsulated leadfrom the conductor cable 23 to theseismometer. Enlarged openings'31 attheffloat ends hold resilientsleeves 38 which inhibit sharp bending and breakage of the cable whereit enters or leaves the float.

In Figure 4 is shown a suitable seismometer assembly consisting of aseismometer 40', gimbalmounted in a frame M which is set in a pair ofanti-friction bearings 42 and 43. The electrical leads from seismometerare brought out through bearing 43 to a pair of slip-rings 44 and 45which are contacted by brushes 46 and 41 connected suitably to insulatedleads in the cable 23 through the splice and insulated lead extendingthrough passage 36. This seismometer assembly is housed in a water-tightcylindrical housing 48 which fits into the cavity in float 25. Beingmounted with the center of gravity of the system at at below the axis ofbearings 42 and G3, the

seismometer is free to rotate about this axis, and it therefore remainsupright at all times despite tric metersatthe recording locationarenoted' any; possible: rotation ofzfloatx25: about cable 23 asccan:axis; Rotations. of. seismometer 40 about axes: perpendicular to cable..23 are generally negligible because of the cable. tension and thestreamlined construction of :floats 24 and 25.

A typical cable-supportmgfloat 24 is shown in Figurezz 5... This maybeconstructed like seismometerfloati25, rounded orpointed at the endsand:intwo :halves. provided with longitudinal slots and: clampedtogether around cable 23 as by meansiofv encircling metal bands 50, orfastened togetherbyboltsor screws.

'Ilie: planiview off Figure 6 and-the sectional viewxcfEigure '7,showing the spread of. Figure-6 inshooting position ina body of water,illustrate aacomplete spread and an' auxiliary shooting vessel in therelative positions occupied'during operations. For' simplicity, thecable floats 24 have been omitted from these figures and only the.seismometer floats 25are shown. Tenseismometers'are employedspaceduniformly apart by distances of 200 "to 250 feet except for thetwo seismometers at each end of the spread, which are spaced between50'and feet apart; Each of these end seismometer pairs is designedtooperate and submerge as a unit, and its depthof submergence is indicatedby a remote-indicating the spread slows down and comes to a stopat'thedesired position relative to the shot point. Seis mometers 25 then beginto submerge slowly: Vessel 54 places an explosive charge 55 and pays outa firing'line 55,the charge beings'uitably" supported by floats orotherwise either above 'or below the water surface, and beingoppositethe center and normally o'ifset from the line of spread 22 -bythe distance d of the order of 300 feet. As soon'ias the seismometers,which have continued' tosink, reach the desired depth-immediately below'the zone of surface Water noise-which nor mally occurs in from one totwo minutes, but may requiremore or'less time in thepresence of aid ing"or opposing water currents-charge 55 is detonated and the seismic waverecord is-made:

The readings-of the depth gauges locatedin each of theu'nits 52appearing ondirect-reading elecbutare not necessarily recordedautomatically on the record. As soon as the record is completed,

thesurfacewhile traveling'to the next shot point.

In the embodiment illustrated in Figure 8, the buoyancy of the varioussections of the spread is distributed somewhat differently from thatwhich has just been described. The cable float 24a, midway between twoseismometer floats :25, is the point of the spread having the greatestpositive buoyancy, as in the previous embodiment. However, instead ofmaking the seismometer float 25 the most negatively buoyant section, itis preferably neutral or even slightly positively buoyant, and two ofthe cable floats 241) located near and on each side of the seismometerfloat 25 are made negatively buoyant by an amount suflicient to makefloat 25 submerge. The two sagging cable portions extending from thepositively buoyant float 24a through the negatively buoyant float 24b tothe neutral or positively buoyant seismometer float-25, and thence tothe next buoyant float 24a, offer an increased length of path for theattepuation of longitudinal vibrations transmitted along the cable 23.

The operation of this arrangement is the same as forthat previouslydescribed, except that the seismometer floats 25 may submerge somewhatmore slowly than in the first embodiment. The floats 24b in thisarrangement submerge most rapidly and pull down seismometer float 25 ata somewhat slower rate. By havingthe seismometer float 25 positivelybuoyant, it is more certain that during towing it will run at thesurface of the wa erand hence be out of danger. Furthermore, as for agiven depth of submergence of the floats 25 Ithere 'must be a greatershortening of the length of the entire spread, it is apparent that therelative buoyancies of the units are less critical than in the previousembodiment.

While my invention has been thus described in terms of the foregoingspecific embodiments, it is to be understood that these are merely forpurposes of illustration, and that many other modi-- flcations of theinvention will occur to those skilled in the art.

- example, any number of seismometers.

other thanten, and. any other spacing pattern oftseismometers within thespread could be employed. Similarly, a wide variety of relativearrangements of spread and shot-point are possible.- Likewise, althoughonly single floats, as 24 a or 24b, have been described as being madepositively or negatively buoyant, the desired buoyancy properties couldbe distributed over severaladjacent floats making a whole section of thecable float or submerge. The invention, therefore, is not to beconsidered as limited to the specifically described details, but is tobe ascertained from the scope of the appended claims.

I claim: I

l. A detector spread for marine seismic surveying comprising a-tensionand conductor cable, a plurality of seismic-wave detectors spaced alongsaidcable, a plurality of buoyant supporting .mea-ns spaced along saidspread, the overall buoyancy of said spread having a positive value withthe portions of maximum positive buoyancy being between and remote fromsaid detectors, and the buoyancy of the portion of said spreadassociated with each of said detectors having a small negative valueeffective to cause slow and complete submergence of ,each detector andthe adjacent portions of the spread below the water surface.

A towable seismometer spread for marine along said cable-and connectedto respective conductors therein, buoyant supporting means for saidcable and said seismometers distributed at points'therealong andproviding a small overall positive buoyancy for said spread, thebuoyancy of said suporting means being positive-midway between theseismometer locations in said spread and slightly negative near saidseismometer locations, and the buoyancy of the remainder of said spreadbeing substantially neutral, whereby said seismometers are near thewater surface during forward motion of said spread and slowly submergealong with substantially all of said cable except that midwaybetween'said seismometers when said forward mo- 7 either side ;of saidseismometers having a negativebuoyancy sufflcient to cause saidseismometers and adjacent portions of saidcable to submerge slowly whenthe forward motion of said spread substantially ceases.

4. Atowable seismometer spread for marine seismic surveying comprising atension'and conductor'cable, a plurality of seismometers spaced alongand connected to conductors in said cable, a plurality of spaced floatssupporting said cable and said seismometers, said floats, cable-andseismometers having collectively asubstantial- 1y neutral buoyancy,except for the floats midway'betweensaid seismometers which have apositive buoyancy of at least 10 ounces, and the floats associated withsaid seismometers which have a negative buoyancy of between 4 and- 8ounces.

v WADDIE 'W..DOOLITTLE.

' REFERENCES orrED I The following references are of record in the flleof this patent:' v

U 'STATEs PATENTS.- Number 7 Name Date 2,203,894 Cooke June 11, 1940-2,241,428 Silverman- May 13, 1941 2,283,200 -Flude May -19, 1942,2,449,085 1948- Peterson Sept. 14,

